Method and apparatus for operating traveling spark igniter at high pressure

ABSTRACT

An ignition circuit and a method of operating an igniter (preferably a traveling spark igniter) in an internal combustion engine, including a high pressure engine. A high voltage is applied to electrodes of the igniter, sufficient to cause breakdown to occur between the electrodes, resulting in a high current electrical discharge in the igniter, over a surface of an isolator between the electrodes, and formation of a plasma kernel in a fuel-air mixture adjacent said surface. Following breakdown, a sequence of one or more lower voltage and lower current pulses is applied to said electrodes, with a low “simmer” current being sustained through the plasma between pulses, preventing total plasma recombination and allowing the plasma kernel to move toward a free end of the electrodes with each pulse.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.13/222,298 filed Aug. 31, 2011, now allowed, which in turn is acontinuation of U.S. patent application Ser. No. 12/313,927 filed Nov.26, 2008, now pending, which in turn is a continuation of U.S. patentapplication Ser. No. 11/407,850 filed Apr. 19, 2006, now U.S. Pat. No.7,467,612, issued Dec. 23, 2008, which claims the benefit, under 35 USC119(e) of prior U.S. provisional patent application Ser. No. 60/672,892,filed Apr. 19, 2005, the entire contents of all of which areincorporated herein by reference in their entireties.

BACKGROUND Field

This invention relates to the fields of plasma generation, ignitions,and internal combustion (IC) engines. In particular, it relates, but isnot limited, to ignition methods and ignition apparatus for use therein;and, specifically, to ignition methods and apparatus for variousapplications, including but not limited to, high pressure engines. Moreparticularly, some aspects relate to the delivery of discharge currentto traveling spark igniters in order to maximize their performance andlongevity, especially in internal combustion engines operating at highpressures.

Discussion of Related Art

For a variety of reasons, there is interest today in increasing thepressures in internal combustion engines and similar combustionenvironments, with a concomitant need for ignition sources capable ofoperating in these environments. For example, automobile companies andmanufacturers of internal combustion engines would like to be able toprovide vehicles which have IC engines which operate at much higherpressures than conventional internal combustion engines. To date,however, there has not been an effective and practical ignition systemfor such engines. Among other concerns are longevity of igniters (sparkplugs) and reliability of igniter firing.

The traveling spark igniter (TSI) is a device that has been discussed asa promising spark plug replacement for internal combustion engines, butpreviously not for high pressure engines. TSIs have, for example, beenshown in a number of prior patents including, for example, U.S. Pat.Nos. 6,321,733 and 6,474,321, both assigned to the same assignee as thisinvention and incorporated by reference in their entireties for theirexplanations of TSI devices and ignition systems.

Briefly, a TSI-based ignition system provides a large plasma kernelwhich is propagated along the igniter's electrodes by Lorentz force(along with thermal forces, to lesser degrees) and propelled into acombustion chamber. The Lorentz force acting on the ignition kernel(i.e., plasma) is created by way of the discharge current in the plasmainteracting with a magnetic field caused by that same current in theelectrodes of the igniter. The magnitude of the Lorentz force isproportional to the square of that current. In engines operating atnormal pressures (i.e., a maximum of about 120 psi), traveling sparkigniters provide significant advantages over conventional spark plugsdue to the large plasma volume they generate, typically some 100-200times larger than in a conventional spark plug, for comparable dischargeenergy. Increased efficiency and reduced emissions are attainable.

For higher engine operating pressures, however, the breakdown voltagerequired for initiating the discharge between the electrodes of theigniter is significantly higher than in engines operating atconventional pressures. This creates problems for TSIs, as for any sparkplug. The electrodes in a TSI, as in a conventional spark plug, aremaintained in a spaced apart relationship by a member called anisolator, which is formed of an insulating material such as a ceramic.The higher breakdown voltage causes problems for both the isolator andthe electrodes.

Along the surface of the isolator running between the electrodes, thebreakdown voltage is lower than it is further along the electrodes in aTSI, or in any conventional spark plug with a similar gap between theelectrodes. Indeed, this difference in breakdown voltages variesdirectly with increasing pressure in the combustion chamber.Consequently, although the breakdown voltage along the isolator surfaceincreases with pressure, that increase is less than the increase in thebreakdown voltage between the exposed part of the electrodes away fromthe isolator surface. When breakdown occurs (as a result of which theresistance through the plasma rapidly drops), the current rises rapidlyand a very large current is conducted in the forming plasma along theisolator surface, thus giving rise to the Lorentz force acting on theplasma. Such rapidly rising current, though, creates not only a veryhigh temperature plasma, but also a powerful shock wave in the vicinityof the surface of the isolator. The larger the current, the more rapidthe plasma expansion and the resulting shock wave. These combinedeffects can cause deformation and/or breakage of the isolator.

Additionally, the high current produces very rapid erosion of theelectrodes in the vicinity of the isolator surface, where they areattacked by the high current, thermal heating and thermionic emissionthat results therefrom.

Similar problems have been manifest with igniters based on theUniversity of Texas “railplug” design which generates a Lorentz force ina plasma traveling along a high aspect ratio discharge gap (ascontrasted with a TSI, which has a low aspect ratio discharge gap).

Although both the railplug and the TSI generate significant plasmamotion at relatively low pressures, when the combustion chamber pressureis increased to a high pressure, the plasma behaves differently and itis this difference in behavior that leads to unsatisfactory results. Ina low pressure environment, the force exerted on the plasma by thepressure is relatively small. The plasma moves easily along theelectrodes in response to the Lorentz force. As the ignition chamberpressure is increased, however, that pressure provides a force ofsignificant magnitude that resists the Lorentz force and, thus, plasmamotion. Consequently, the plasma tends to become more concentrated, andto collapse on itself; instead of having a diffused plasma cloud, a verylocalized plasma—an arc—is formed between the electrodes below a certaincurrent threshold. This arc, though occupying a much smaller volume thanthe plasma cloud of the low-pressure case, receives similar energy. As aresult, the current density is higher and at the electrodes, where thearc exists, there is a higher localized temperature and more powerdensity at the arc-electrode interfaces. That is, the current density isquite high at those interfaces, producing more localized heating of theelectrodes than in the low pressure environment. The localized heatingof the electrodes, in turn, produces thermionic emission of electronsand ions. The observed effect is that the arc appears to “attach” itselfat relatively fixed locations on the electrodes, producing erosion ofthe electrodes as the entire discharge energy is deposited at the“attachment point;” this is to be contrasted with the low pressureenvironment where a lower density, diffused area of plasma contact movesalong the electrodes without significantly damaging them.

Concurrently, the plasma, affected by the Lorentz and thermal forces,bows out from the arc attachment points. This causes the magnetic fieldlines to no longer be orthogonal to the current flow between theelectrodes, reducing the magnitude of the Lorentz force produced by agiven current. So, in addition to the other problems, there is a loss inmotive force applied to the plasma.

Overall, there is a reduction in plasma motion as compared with thelower pressure environments, and dramatically increased electrode wearat the arc attachment points.

Accordingly, a variety of needs exist, including needs for plasmagenerators, in general, needs for improved ignition systems, needs forignition systems for use in internal combustion engines and needs for anignition system and method which generates a large ignition kernel andwhich is usable with high pressure engines, and is commerciallypractical.

If a traveling spark igniter is to be used in a high pressure combustionenvironment, a need further exists to overcome the above negativeeffects on the isolator material and electrodes of the igniter. See U.S.Pat. Nos. 5,704,321, 6,131,542, 6,321,733, 6,474,321, 6,662,793, and6,553,981, for example, incorporated by reference herein. That is, aneed exists for an igniter and ignition system for use in high pressurecombustion engines, wherein the isolator and electrodes exhibitsubstantial lifetimes (preferably comparable to that of conventionalspark plugs in low pressure engines) without being destroyed by thedischarge process. Desirably, such a traveling spark igniter andignition system will be usable and useful in internal combustion enginesoperating not only at high and very high pressures (i.e., severalhundred psi), but also at lower, conventional pressures.

SUMMARY

The above and other needs are addressed, and advantages provided, with anew method, and corresponding apparatus, for generating and sustaining aplasma, operating a traveling spark igniter and providing an ignitionfor internal combustion and other engines, particularly high pressureinternal combustion engines. Typically, a high initial breakdown voltageis applied to the igniter to initiate a plasma kernel in a plasmainitiation region of the igniter, but preferably at a current lower thanthat previously employed with TSI ignitions, as the breakdown currentneed not produce a large Lorentz force. After the breakdown currentpulse, various mechanisms may be employed to prolong the plasma whilerecombination is occurring and to allow the plasma to become easilydetached (or detachable) from the initiation region (typically, on oradjacent the surface of an isolator between the igniter electrodes.Before the plasma has a chance to recombine completely, the current isturned on again to provide a short follow-on pulse of energy (preferablyat a current substantially less than that of the breakdown pulse). Thefollow-on current pulse generates a corresponding pulse of Lorentz forceto move the plasma away from its previous location, further along theelectrodes of the igniter. A number of such follow-on pulses may beprovided, with an “off” interval between successive pulses, during whichinterval one or more mechanisms prolong the plasma and allow onlypartial recombination of the plasma. This is called “simmering.” Priorto total recombination of the plasma, the next follow-on pulse ofcurrent “kicks” the plasma even further along the electrodes; and thefinal follow-on pulse ejects the plasma from the electrodes. Onemechanism for producing simmering is to reduce the current through theigniter to a relatively low (but non-zero) level, called a “simmercurrent.” Alternatively, if a summer current is not applied, similareffects may be obtained by using any of a number of other techniques forprolonging recombination and preventing “total” recombination of theplasma kernel by the time the next follow-on pulse arrives. For example,the follow-on pulses may be timed and possibly even waveform-shaped tomore closely follow each other so that only partial recombination occursbetween pulses; or each follow-on pulse may be preceded by a highsub-breakdown voltage; or the plasma may be excited by RF or laserenergy. That is, numerous ways are contemplated of preventing totalplasma recombination. By “total” in reference to recombination is meantthat the plasma effectively has been extinguished and high energy isneeded to reignite it.

The invention is manifested in several ways, or aspects, and exampleimplementations are presented below. Other ways of practicing theinvention will become apparent to those skilled in the art. The variousaspects may be practiced alone or in any of many combinations, all ofwhich cannot be reasonably enumerated here. It is intended that featuresof various embodiments be practiced in combinations other than thoseillustrated, not all features being shown in connection with allembodiments, for brevity.

Aspects of the invention include the following, at least: A method ofplasma generation, comprising applying a high voltage to an igniter,said high voltage being of amplitude sufficient to cause breakdown tooccur between the electrodes, resulting in a high current electricaldischarge in the igniter in an initiation region, and formation of aplasma kernel adjacent said initiation region; and following breakdown,applying to said electrodes a sequence of at least two relatively lowervoltage follow-on pulses, whereby the plasma kernel is forced to movetoward a free end of said electrodes by said follow-on pulses.

A method of plasma generation, comprising applying a high voltage to anigniter, said high voltage being of amplitude sufficient to causebreakdown to occur between the electrodes, resulting in a high currentelectrical discharge in the igniter in an initiation region, andformation of a plasma kernel adjacent said initiation region; andfollowing breakdown, applying to said electrodes a sequence of one ormore relatively lower voltage follow-on pulses of current sufficientlylow as to maintain a diffuse attachment of the current arc to theelectrodes, whereby the plasma kernel is forced to, and can, move towarda free end of said electrodes under the influence of said follow-onpulses.

The initiation region may be on or adjacent the surface of an isolatordisposed between said electrodes. A current of the follow-on pulses, foran internal combustion engine, may be between about 3 and 450 Amperes.The method may include preventing total kernel recombination of theplasma prior to at least one follow-on pulse. This may be done invarious was, including between pulses of the sequence, maintaining asimmer current between the igniter electrodes sufficient to preventtotal recombination of the plasma kernel. It also may include, in aninterval between follow-on pulses, for at least part of said intervalmaintaining a voltage across electrodes of the igniter below a breakdownvoltage but sufficient to sustain enough current to prevent totalrecombination before the end of the interval. The igniter may be atraveling spark igniter. Successive pulses in said sequence areseparated by intervals of about 2-600 microseconds and preferably about20-250 microseconds, most preferably 50-100 microseconds. Each of saidfollow-on pulses may have a maximum amplitude of about 3-450 Amperes.The amplitudes may not be uniform. The follow-on pulses may have amaximum amplitude of about 20-120 Amperes, which may not be uniform.Each of said follow-on current pulses preferably may have an averageduration of less than about 200 microseconds, which may not be uniform.The follow-on pulses may have an amplitude of about 10-5000 V andpreferably about 20-275 V. The follow-on pulses need not all have thesame polarity of voltage and current and the currents of the follow-onpulses need not be constant.

A fuel ignition method, comprising applying a high voltage to an igniterin the presence of a combustible fuel, said high voltage being ofamplitude sufficient to cause breakdown to occur between the electrodesof the igniter, resulting in a high current electrical discharge in theigniter in an initiation region, and formation of a plasma kerneladjacent said initiation region; and following breakdown, applying tosaid electrodes a sequence of two or more relatively lower voltagefollow-on pulses, whereby the plasma kernel is forced to move toward afree end of said electrodes by said follow-on pulses. The initiationregion may be on or adjacent the surface of an isolator disposed betweensaid electrodes. The igniter may be in an internal combustion engine. Acurrent of the follow-on pulses for a gasoline-fueled internalcombustion engine, may be between about 3 and 450 Amperes. Preferably,said method includes preventing total kernel recombination of the plasmaprior to a follow-on pulse.

Preventing total recombination may include, between pulses of thesequence, comprises maintaining a current (termed a simmer current)through the plasma kernel sufficient to prevent total recombination ofthe plasma kernel. Preventing total recombination of the plasma kernelalso may include, in an interval between follow-on pulses, for at leastpart of said interval maintaining a voltage across electrodes of theigniter below a breakdown voltage but sufficient to sustain enoughcurrent through the plasma to prevent total recombination before the endof the interval.

Follow-on pulses need not all have the same polarity of voltage andcurrent, which need not be constant.

The igniter may be in an internal combustion engine in which there is arelatively high pressure at the time of ignition.

The methods may further include, after a follow-on pulse, re-triggeringor re-striking the plasma kernel at a time an ionization level of theplasma kernel has fallen below a desired level, with a current and at arelatively low voltage sufficient to cause the plasma kernel to growbefore total recombination occurs, followed by a next follow-on pulse.

The methods also may include simmering the plasma kernel between atleast some follow-on pulse pairs.

An ignition circuit for powering an igniter in an internal combustionengine, comprising means for providing a high voltage capable causing anelectrical breakdown discharge, at a high current, between electrodes ofan igniter, in an initiation region between said electrodes, when saidigniter is disposed in a fuel-air mixture of an engine, whereby a plasmakernel is formed in said region by said discharge; and means forproviding a sequence of one or more relatively lower voltage and lowercurrent pulses having voltage and current amplitude and timingsufficient to force the plasma kernel to move toward a free end of saidelectrodes by said lower voltage, lower current pulses. The means forproviding a high voltage capable of causing electrical breakdowndischarge may include a high voltage, low inductance ignition coilhaving a primary winding and a secondary winding, the secondary windinghaving a lead for connection to one electrode of an igniter, and acircuit for triggering a signal in the primary winding to induce a highvoltage pulse in the secondary winding. The means for providing asequence of relatively low voltage pulses may comprise a relatively lowvoltage source and, for each said pulse, a capacitor charged by therelatively low voltage source and a pulse transformer having a secondarywinding connected to said lead and a primary winding through which thecapacitor is discharged in response to a trigger signal, inducing saidpulse in said lead. The ignition circuit may further include means forproviding to the igniter, in an interval between the breakdown dischargeand a first follow-on pulse a simmer current sufficient to prevent totalrecombination of the plasma kernel in said interval. It also may includemeans for providing to the igniter, in an interval between eachsuccessive pair of follow-on pulses a simmer current sufficient toprevent total recombination of the plasma kernel in said interval. Theignition coil preferably includes a saturable core on which the primaryand secondary windings are formed and the core substantially saturateswhen said electrical breakdown occurs, whereby the secondary windingthereafter has substantially reduced inductance.

An ignition circuit for powering an igniter in an internal combustionengine, comprising a high voltage pulse generator which generates on anoutput for connection to an igniter a pulse whose maximum voltage, whendelivered to the igniter, is capable causing a breakdown discharge andconsequent high current between electrodes of the igniter, in aninitiation region between the electrodes, when said igniter is disposedin a fuel-air mixture, whereby a plasma kernel is formed adjacent saidsurface by said discharge; and a lower voltage pulse generator whichgenerates on the output a sequence of one or more relatively lowervoltage and lower current follow-on pulses having voltage and currentamplitude and timing sufficient to force the plasma kernel to movetoward a free end of said electrodes by said lower voltage, lowercurrent pulses. There may also be included a simmer current source whichsupplies on the output line, in an interval between the breakdowndischarge and a first follow-on pulse a simmer current sufficient toprevent total recombination of the plasma kernel in said interval. Aswell, there may be a voltage source which maintains between follow-onpulses, for at least a portion of an interval between said follow-onpulses, a voltage on the igniter electrodes below a breakdown voltagebut sufficient to prevent total recombination of the plasma kernelduring said interval.

An ignition circuit substantially as shown and described in the drawingfigures, particularly any of FIGS. 8-10 .

The ignition circuit also may include means operable after a follow-onpulse, for re-triggering or re-striking the plasma kernel at a time anionization level of the plasma kernel has fallen below a desired level,with a current and at a relatively low voltage sufficient to cause theplasma kernel to grow before total recombination occurs, followed by anext follow-on pulse.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings are not intended to be drawn to scale. In thedrawings, each identical or nearly identical component that isillustrated in various figures is represented by a like numeral. Forpurposes of clarity, not every component may be labeled in everydrawing. In the drawings:

FIG. 1 is a schematic illustration, in cross section, of a prior arttraveling spark igniter, illustrating the principle of its operation;

FIG. 2 is a part-schematic, part-block diagram of a typical prior artignition circuit for the TSI of FIG. 1 ;

FIG. 3 is a generalized representation of the voltage between theelectrodes of an igniter as shown in FIG. 1 , using an ignition circuitof the type shown in FIG. 2 ;

FIG. 4 is a diagrammatic illustration of the creation of a plasma cloudby a current pulse in a TSI, and the subsequent collapse of the plasma,in a TSI operating in a high pressure environment;

FIG. 5 is a waveform of an example of a drive current applied to a TSIin accordance with the teachings of the present invention;

FIGS. 6 and 7 are diagrammatic illustrations of the motion of the plasmacloud of FIG. 4 in a TSI which is operated in accordance with theprinciples exemplified in the waveform of FIG. 5 ;

FIG. 8 is a simplified schematic circuit diagram for an example of anignition drive circuit usable to generate a current drive waveform for aTSI as taught herein, including, for example, the waveform or drivesignal of FIG. 5 ;

FIG. 9 is a simplified part-block, part schematic circuit diagram ofanother embodiment of an ignition circuit for generating an ignitiondrive to a TSI as taught herein;

FIG. 10 is a simplified part-block, part schematic circuit diagram ofyet another embodiment of an ignition circuit for generating an ignitiondrive to a TSI as taught herein; and

FIG. 11 is a simplified part-block, part schematic circuit diagram of astill further embodiment of an ignition circuit for generating anignition drive to a TSI as taught herein.

DETAILED DESCRIPTION

Herein are explained in greater detail numerous aspects of theinvention; the problems addressed by the invention, in greater detailthan above; and a single embodiment of an example of an ignition circuitfor practicing aspects of the invention.

According to a first aspect, there will be shown a method of operatingan igniter in an internal combustion engine, comprising: applying a highvoltage to electrodes of the igniter, said high voltage being ofamplitude sufficient to cause electrical discharge breakdown to occurbetween the electrodes, in an initiation region (e.g., over a surface ofan isolator) between the electrodes, resulting in a high currentelectrical discharge in the igniter, and formation of a plasma kernel inan air or fuel-air mixture adjacent said surface; and followingbreakdown, applying to said electrodes (preferably a simmer current) anda sequence of one or more lower voltage and lower current pulses,whereby the plasma kernel is forced to move toward a free end of saidelectrodes by said lower voltage, lower current pulses.

Between breakdown and a first pulse of the sequence, and between pulsesof the sequence, a current desirably is maintained through the plasmakernel sufficient to prevent total recombination of the plasma.Alternatively, such a current need not be maintained, if the intervalsbetween breakdown and the first pulse of the sequence, and betweenadditional follow-on pulses of the sequence, are sufficiently short,such that total recombination does not occur prior to the start of suchpulses. (If total recombination occurs, then a high breakdown voltage isneeded to restart the plasma formation process.) If total recombinationis avoided (no matter how) before the start of a follow-on pulse, thefollow-on pulse can be a relatively low current pulse (compared to anumber of previous approaches, but still appreciable) and it will stillprovide a suitable Lorentz force to advance the plasma, and it will,itself, create a current arc that can move along the electrodes. Asanother alternative, recombination can be slowed by imposing arelatively high (but less than breakdown) voltage across the electrodesprior to the start of a follow-on pulse. All three mechanisms facilitatethe establishment of a moving plasma kernel without requiringre-generation of a high energy breakdown condition, reducing thetendency of the current path to “re-attach” to the electrodes at fixedlocations. The number of follow-on pulses varying according to designrequirements and/or operating conditions.

The igniter is preferably a traveling spark igniter.

Desirably, a first pulse of the sequence follows the breakdown dischargeby an interval of from about 2 to about 100 microseconds, preferablyfrom about 10 to about 20 microseconds, but this will depend on therecombination time for a plasma in the particular kind of fuel mixturebeing employed. Desirably, each of said follow-on pulses has a maximumamplitude of about 5-200 Amperes. But the amplitudes need not beuniform. Preferably, said lower voltage, lower current pulses have amaximum amplitude of about 25-105 Amperes, and more preferably about40-80 Amperes. The pulses may have a duration of from about 2 to about200 microseconds. Successive pulses in said sequence preferably areseparated by intervals of about 10-500 microseconds and even morepreferably, 40-120 microseconds, but the intervals may not be uniform.In terms of voltage, each of said pulses typically may have an amplitudeof about 50-5000 V and, more preferably, about 300-500 V. All pulsesneed not have the same polarity of voltage or current; and neither thevoltage nor the current in a pulse need be constant. The foregoingnumbers are all representative only and are not intended to reflect anyinherent limits on the invention. Other ranges may be employed inappropriate embodiments.

These numbers may be useful, though, as an aid to identifyingdifferences with other ignition systems and methods.

The invention is intended for use in high pressure engines, but is notso limited.

According to a related aspect, an ignition circuit is provided forpowering an igniter in an internal combustion engine, the circuitcomprising means for providing a high voltage capable of causing abreakdown discharge, at a relatively high current (but preferably lowerthan prior TSI ignitions have used), between electrodes of an igniter,and in an initiation region (e.g., on or over a surface of an isolatorwhich separates the electrodes), when said igniter is disposed in afuel-air mixture, whereby a plasma kernel is formed adjacent saidsurface by said discharge; and means for providing a sequence of one ormore relatively lower voltage and lower current follow-on pulses havingvoltage and current amplitude and timing sufficient to create Lorentzforce pulses causing the plasma kernel to move toward a free end of saidelectrodes by said follow-on pulses. The means for providing a highvoltage capable of causing breakdown may include a high voltage, lowinductance ignition coil having a primary winding and a secondarywinding, the secondary winding having a lead for connection to oneelectrode of an igniter, and a circuit for triggering a signal in theprimary winding to induce a high voltage pulse in the secondary winding.

The means for providing a sequence of relatively lower voltage (i.e.,sub-breakdown voltage) pulses may comprise a low voltage source and, foreach said pulse, a capacitor charged by the low voltage source and apulse transformer having a first winding connected to said lead and asecond winding through which the capacitor is discharged in response toa trigger signal, inducing said pulse in said lead. The ignition circuitmay further include means for providing to the igniter, in an intervalbetween the breakdown discharge and a first lower voltage pulse a simmercurrent sufficient to prevent total recombination of the plasma kernelin said interval. It also may include means for providing to theigniter, in an interval between successive follow-on pulses a simmercurrent sufficient to prevent total recombination of the plasma kernelin said interval. Alternatively the means for providing a sequence ofrelatively low voltage pulses includes means for providing pulsesseparated in time by an interval sufficiently short that totalrecombination of the plasma kernel does not occur in said interval. Asanother alternative, the means for providing a sequence of relativelylow voltage pulses may comprise a means for preceding each suchfollow-on pulse by a high, sub-breakdown voltage.

According to a further aspect, an ignition circuit is shown for poweringan igniter in an internal combustion engine, the circuit comprising ahigh voltage pulse generator which generates on an output for connectionto an igniter a pulse whose maximum voltage, when delivered to theigniter, is capable causing a breakdown discharge, at a high current, inan initiation region between electrodes of the igniter (e.g., adjacent asurface of an isolator which separates the electrodes), when saidigniter is disposed in a fuel-air mixture, whereby a plasma kernel isformed adjacent said surface by said discharge; and a low voltage pulsegenerator which generates on the output a sequence of one or more lowervoltage and lower current pulses having voltage and current amplitudeand timing sufficient to force the plasma kernel to move toward a freeend of said electrodes by said lower voltage, lower current pulses. Theignition circuit may further include a simmer current source whichsupplies on the output, in an interval between the breakdown dischargeand a first lower voltage pulse, a simmer current sufficient to preventtotal recombination of the plasma kernel in said interval.Alternatively, the circuit may include a follow-on pulse generator thatsupplies, on the output, follow-on pulses which follow each other soclosely (i.e., are separated by a sufficiently short interval) thattotal recombination of the plasma does not occur in the interval betweensuch pulses. As another alternative, the circuit may include a pulsesource providing a sequence of relatively low voltage pulses and a highvoltage source which provides, preceding each such follow-on pulse, asub-breakdown high voltage sufficient to delay total recombination suchthat total recombination has not occurred when the relatively lowvoltage pulse starts.

Thus, this invention is not limited in its application to the details ofconstruction and the arrangement of components set forth in thefollowing description or illustrated in the drawings. The invention iscapable of other embodiments and of being practiced or of being carriedout in various ways. Any embodiments are presented by way of exampleonly. Also, the phraseology and terminology used herein is for thepurpose of description and should not be regarded as limiting. The useof “including,” “comprising,” or “having,” “containing,” “involving,”and variations thereof herein, is meant to encompass the items listedthereafter and equivalents thereof as well as additional items.

It is useful, now, to attempt to better understand the problemsencountered when one attempts to operate an igniter in a high pressureengine. A traveling spark igniter (TSI) is an ignition device which isin the nature of a small plasma gun. A typical TSI is illustrated inFIG. 1 , taken from U.S. Pat. No. 6,321,733. An isolator (e.g., ceramic)material 14 maintains electrode spacing. A plasma 16 is created alongthe surface of the isolator, due to a high voltage breakdown processoccurring there. As the discharge current passes through the plasma, thetemperature and volume of the plasma increase, leading to a furtherdecrease in plasma resistivity and resistance. This increases thecurrent in the plasma, which is limited primarily by the impedance ofthe electrical discharge circuit that produces the current supplied tothe igniter.

A typical ignition circuit for operating a TSI is shown in FIG. 2 ,which is also taken from U.S. Pat. No. 6,321,733. The circuit consistsof two main parts: (1) a conventional ignition system 42 and (2) afollow-on current generator comprising capacitors such as 46 and 48, alow voltage power supply 44 and diode 50. The conventional ignitionsystem 42 provides a high voltage for creating a breakdown (at a highcurrent) in the spark gap along the isolator surface 56 between theelectrodes 18 and 20, to form an initial plasma in the gaseouscombustion mixture near that surface. The follow-on current generatorprovides a current through the initial plasma, in the spark gap, afterbreakdown discharge, forming a much larger plasma volume. Resistor 54may (but need not) be used to limit the maximum current from capacitor48. A typical voltage discharge profile (not to scale) is shown in FIG.3 , taken from U.S. Pat. No. 6,474,321.

The conventional ignition system 42 initiates discharge in the dischargegap at time t=t₀. As a result, the voltage in a secondary coil in thehigh voltage (HV) ignition transformer therein rises until it reachesthe breakdown voltage in the spark gap at t=t₁. After breakdown occursat t=t₁ the voltage across the discharge gap drops rapidly to value ofabout 500 volts or less at t=t₂, corresponding to low plasmaresistivity. The voltage is substantially constant until a time t=t₃,when just about all the energy from capacitors 46 and 48 has beentransferred, following which the voltage and current rapidly diminish toa near-zero value at time t=t₄. For simplicity, we shall assume that theinterval from t₃ to t₄ is negligibly short. The interval Δt=t₃−t₂ isrelated to the energy stored in capacitors 46 and 48 as well as thevoltage of the follow-on current through the discharge gap afterbreakdown occurred. The following energy balance equation relates thevariables:

${\frac{1}{2}{C\left( {V_{t_{2}}^{2} - V_{t_{4}}^{2}} \right)}} = {\int\limits_{t_{2}}^{t_{4}}{{V(t)}{i(t)}{dt}}}$

where V(t) is the voltage as a function of time, between the electrodesdefining the discharge gap, such voltage having an initial value V_(t) ₂at time t₂ and a final value V_(t) ₄ ≈0 at t>t₄, i(t) is the current inthe spark gap as a function of time and C is the sum of the dischargingcapacitance (here, the sum of capacitances of capacitors 46 and 48). Inthe time interval Δt=t₃−t₂, one can assume, as a first approximation,that V(t)≈V₀ and is roughly constant, therefore, V_(t) ₂ ²−V_(t) ₄ ²≈V₀². If one further assumes that the plasma resistivity is constant, onecan make the assumption i(t)≈i₀. One can use these simplifyingassumptions to obtain a basic relationship between Δt (Δt≈t₄−t₂ becauset₄−t₃<<Δt) and the circuit parameters described by C, V₀, and i₀:

Δt=CV ₀/2i ₀

This simple relationship provides information about pulse duration as afunction of capacitance and average current i₀ during discharge, for agiven operating (relatively low) voltage V₀ on the capacitors. For agiven energy provided to the igniter (hence, given V₀ and C), thisrelationship teaches that for current i₀ to increase, the pulse durationΔt has to decrease. However, increasing current i₀ also increases theLorentz force F_(L). Increasing the Lorentz force moves the plasma awayfrom the isolator surface faster, toward the end of the electrodes, intothe combustion chamber of the engine. Pressure in the combustionchamber, however, provides a countervailing pressure force F_(p) in theigniter. Force F_(p) works against the Lorentz force preventing thespeed of the plasma from increasing above some limiting value,independent of the length

of the electrodes (i.e.,

is the distance between the surface of the isolator and the free end ofelectrodes facing into the combustion chamber).

The net force available to move the plasma is the difference between theLorentz force F_(L) and the pressure force F_(p) (assuming one canignore the thermal force on the plasma as it is significant only at theearlier stages of plasma propagation and diminishes quickly as theplasma moves away from the isolator surface). It is useful to develop amodel of the forces in order to understand how to overcome the pressureforce. The Lorentz force F_(L) can be represented as a magnetic pressurePB on the plasma, given by the well-known relationship P_(B)=B²/8π,multiplied by the effective plasma surface area,

:

$F_{L} = \frac{B^{2}}{8\pi}$

The gas pressure force Fp can be presented in the form F_(p)=p

, where p is the effective gas pressure from the combustion mixture(facing the plasma during its movement). Hence, one can write theequation for the net force governing plasma movement can be presentedas:

(F _(L) −F _(p))=

,d

/dt,

where

is plasma velocity and

is plasma mass. In turn, plasma mass can be presented as the product ofplasma mass density ρ_(pl) and plasma volume

=

, where

is a fraction representing the portion of the electrode length occupiedmomentarily by the plasma.

The net force equation can be simplified, and useful relationshipsderived from it, by making some rough assumptions. One can assume thatthe plasma volume, after its formation, is constant as the plasmapropagates along the electrodes; thus,

,

and ρ_(pl) are constant and forces F_(L) and F_(p) are also constant.Then, by integrating one obtains:

(F _(L) −F _(p))Δt≈

,

where it was assumed that the initial plasma velocity υ_(t2) was muchsmaller than its final velocity,

.

Replacing F_(L) by B² where B=√{square root over (8πα)} i and α is aconstant coefficient, and F_(p) as above, we obtain

(αi ₀ ² −p)Δt=

.

Because ½ Δt

≈

, we can write

${\Delta t} = {\frac{1}{i_{0}}\left( \frac{2\ell\rho_{p\ell}\Delta\ell_{\ell p}/\alpha}{1 - {p/\alpha{i}_{0}^{2}}} \right)^{1/2}}$

From this equation, one observes that for relatively small pressure(i.e., p<<αi₀ ²), Δt i₀≈constant; and in this range of parameters,increasing i₀ leads to decreasing Δt. Then from the above relationships,one can see that the plasma can be moved faster with increasing i₀without really increasing the discharge energy (of course, this is onlytrue for

≈const.; with increasing i₀.

may also increase, so some additional energy may be required).

However, when it is not true that p<<αi₀ ² (i.e., the assumption fails),then increasing pressure p could lead to p/αi²≥1 and the plasma couldstop moving altogether. In such a case, it will be necessary to increasei>i₀ to the point that

p/αi²<1. This requires a significant increase in energy, though, due toincreased Δt and i.

Recombination processes in the plasma pose a further hurdle. The frontportion of the hot plasma that is in contact with a relatively coldcombustion mixture cools rapidly. The plasma recombination rate at highpressure is a function of plasma temperature, T, that varies as1/T^(3/2). Hence, at low temperature, plasma recombination occurs veryfast at its propagation front where it interacts with the cold gaseousmixture. At high pressures, such recombination rate could be as fast asthe plasma propagation velocity, meaning that the Lorentz force—inducedmovement would be entirely negated by the speed of recombination,effectively causing the plasma to stand still. In such a situation, thenet plasma velocity along the electrodes is substantially zero and theplasma will seem to stay near the surface of the isolator during theentire discharge. The plasma, of course, recombines near the surface ofthe isolator, as well, though at a much slower rate because the gasthere is much hotter than at the plasma's front edge. Consequently,plasma resistivity near the isolator surface is lower than at the frontedge of the plasma and most of the discharge current will beconcentrated in that region, preventing further plasma recombinationnear the isolator.

As shown above, increasing operating combustion chamber pressure lowersthe net motive force on the plasma so it moves more slowly and the timeit takes for the plasma to move to the combustion chamber thusincreases. Therefore, for sufficiently large pressures, the plasma maynever succeed in reaching the end of the igniter.

To prevent the plasma from slowing down so much, the discharge currenthas to be raised, in order to increase the energy being fed into theplasma. The increased energy input, though, is concentrated near theisolator. That is quite problematic. There are thermal stresses imposedon the isolator and shock waves are generated that can damage theisolator. There are also large thermal effects on the portions of theelectrodes near the isolator. Assuming the ignition circuit suppliessufficient energy to create a net force that will effectively move theplasma, then the higher the pressure in the combustion chamber, theworse the negative effects on the isolator and electrodes. Theseconditions decrease isolator and electrode longevity in high pressureenvironments, unless something is done to prevent those negativeimpacts.

The problem of decreasing longevity of traveling spark igniters withincreasing gas (i.e., combustion mixture) pressure is significantlydecreased, or even eliminated, at least in part by decreasing thedifference between the speed of recombination at the front of the plasma(facing the combustion chamber) and the back of the plasma (facing theisolator). By making plasma recombination more symmetrical, asignificant net force on the plasma is directed into the combustionchamber.

FIG. 4 diagrammatically illustrates the problem. A relatively shortfirst current pulse forms a volume of plasma 42, as indicated by thedashed line. During that first pulse, the center of the plasma moves tothe right, away from isolator 14, under the influence of the Lorentzforce. As the pulse is of relatively short duration, neither theisolator surface nor the gas near the surface is heated significantly.Therefore, after the first current pulse ends, the plasma recombines atits back (left) side and its front (right) side fairly symmetrically,leaving a relatively narrow plasma kernel 44. The narrow plasma kernelstill can support an arc, as explained above.

The present invention improves the symmetry of plasma recombination byusing a different approach to energizing the igniter. Several shortcurrent discharge bursts (follow-on pulses) are applied after thebreakdown pulse, between times t₂ and t₃. The follow-on pulses havemoderately high peak current amplitude, but significantly less than thebreakdown pulse. Between the breakdown pulse and the first follow-onpulse, and between follow-on pulses, the (simmer) current preferably ismaintained at a low, non-zero value, to prevent total recombination.

In FIG. 5 , in which the waveform is shown for one example of an ignitercurrent that may be used to excite a TSI as explained above, breakdownoccurs at time t₁ (peak voltage, followed by maximum current) and iscomplete at time t₁*. Beginning at time t₂, a series of (one or more)lower amplitude current pulses 52A-52E (i.e., five pulses, in thisexample, though the number of pulses is variable) are provided betweenthe electrodes of the igniter. The discharge interval ends at time t₃,when the plasma reaches the end of the electrodes. The plasma started atthe isolator at time t₁. The durations τ_(i), τ₂ . . . τ_(n) of therespective pulses 52 and their peak current magnitude, i₀, should bechosen according to igniter design and gas pressure p. In a travelingspark igniter, the pulse durations and magnitudes are selected,preferably, in accordance with the length of the electrodes and the gapbetween them. Experimentation is a satisfactory way, and for the momentprobably the best way, of setting the values of those parameters for agiven igniter design and maximum pressure of its operation.

The time between pulses also depends on igniter design and pressure. Thetime between the breakdown current, when it reaches near-zero level att₁* and the first follow-on pulse 52A, indicated as Δt_(b,1), depends onthe breakdown voltage and the specifics of the isolator between theelectrodes. The simmer current is non-zero and, as such, helps avoidtotal plasma recombination; otherwise, a large voltage (comparable tothe breakdown voltage) would be needed for initiating the next pulse.So, the current is facilitates each subsequence pulse and allows itsformation without the need for an additional breakdown pulse. Thefollowing table provides parameter values which have been found usefulwith TSI igniters operating in a simulated combustion chamber at 400 psipressure:

-   -   Electrode length:        =2.5 mm    -   Peak pulse current: i₀≈20-40 Amperes,    -   Duration of the k-pulse: τ_(k)≈10-20 microseconds,    -   Time between two consecutive pulses k and k+1: Δ_(k,k+1)≈50-100        microseconds,    -   n (i.e., number of pulses) 3 to 4,    -   Simmer current: i_(s)≈1-3 Amperes,    -   Time between end of breakdown and the first follow-on pulse:        Δ_(b,1)≈5-20 microseconds.

These parameters can be significantly different for different design ofspark plugs or values of pressure p. For example, for a TSI similar tothe one in the previous example and operating at pressure p=900 psi,suitable parameters that have been found useful are:

i₀≈60-80 Amperes,

τ_(k)≈20-40 microseconds,

Δt_(k,k+1)≈30-40 microseconds,

n≈7 to 10 pulses,

i_(s)≈3-5 Amperes, and

Δt_(b,1)≈3-10 microseconds.

Though the peak pulse values i₀ and pulse durations τ_(k) and the timesbetween individual pulses Δt_(k,k+1) have been shown as constant, theyneed not be uniform or constant. For example, they could actuallyincrease or decrease as a function of time.

FIGS. 6 and 7 diagrammatically illustrate the operation produced by thispulsed drive scheme. It is assumed the breakdown pulse has alreadyoccurred and the first follow-on pulse is in a position

away from the surface of the isolator, as in FIG. 4 . After a timeinterval Δt_(1, 2) following the first pulse, the next pulse τ₂ occurs,after which the plasma is in a new position

away om the surface of the isolator. With each successive pulse, theplasma kernel is moved to the right and then at the end of the pulse,allowed to recombine (FIG. 6 , showing the plasma position after twopulses), until eventually (FIG. 7 ) the plasma reaches the end of theelectrodes after n current pulses, and is ejected into the combustionchamber. The number of follow-on pulses, n, will depend on the pressurep in chamber, igniter parameters (e.g., the length of the electrodes,the gap between the electrodes, and the shape of the electrodes) andcurrent discharge parameters (e.g., peak values of pulses, theirdurations, the inter-pulse intervals, and minimum current value betweenpulses). Some experimentation may be required to find suitable values.

Although the current pulses are shown as positive pulses in FIG. 5 , itshould be realized that negative pulses can also be used, or alternatingpulses or some other pattern of pluralities. The Lorentz force F_(L) isproportional to the square of the current and is, therefore, independentof current polarity. Additionally, the discharge current pulses, shownas rectangular in FIG. 5 , could have any suitable waveform, such astriangular shape or sinusoidal shape.

As stated above, with increased operating pressure, the breakdown ofvoltage along the surface of the isolator also increases. Increase inbreakdown voltage has a negative impact on the lifetimes of the isolatorand electrodes. Such negative effects can be avoided or significantlyreduced by limiting the breakdown current. For example, introducing aresistor into the high voltage circuit, as described below, limitsbreakdown current without wasting significantly energy when thebreakdown discharge is of short duration in comparison with the totalinterval of follow-on discharge pulses. Limiting the current causes themode of operation to differ substantially from that of prior TSIsystems. In prior TSI systems, such as those shown in U.S. Pat. Nos.6,321,733 and 6,474,321, it was desired that a high breakdown current befollowed immediately by high current from capacitors to create maximumacceleration and plasma speed. The goal was to get the plasma to reachthe end of the electrodes and move into the combustion chamber in asingle discharge pulse. In contrast, in a high pressure environment,plasma motion is small following breakdown. Thus, it is acceptable tolimit the breakdown current since the breakdown current is only used tocreate the plasma near the isolator surface, rather than to actuallyproduce significant plasma motion.

The interval between the end of the breakdown current pulse and thefirst follow-on current pulse, Δt_(b,t1) a depends on the peak value ofthe discharge current. Assuming that a resistor R_(b) is used to achievethis current limiting effect, than the delay time depends on the valueof that resistor, which depends on the applied breakdown voltage which,in turn, depends upon the pressure p. Thus, the value of resistor R_(b)can be chosen to minimize stress on the isolator and electrode wear.

FIG. 8 shows a partial schematic circuit diagram for an example of anelectronic circuit for producing the breakdown pulse and follow-onpulses as depicted in FIG. 5 . In FIG. 8 , circuitry is shown forgenerating only the breakdown pulse and one follow-on pulse. For eachadditional follow-on pulse that is desired, the circuitry 110 enclosedin a dashed line can be replicated and all such circuits can beconnected with the secondary windings of their boost transformers 102 inseries, so that each such circuit will, in turn, deliver one of thesequenced pulses to the igniter. (Note that a parallel arrangement isalso possible.)

A high voltage, for providing breakdown discharge is generated by a highenergy ignition coil 100, triggered by a signal applied at 104 to causeswitching of SCR 104A. Coil 100 may be any suitable ignition coil suchas, but not limited to, coil model 8261 sold by Autotronic ControlsCorporation of El Paso, Tex., d/b/a MSD Ignition. Though usuallyreferred to in the industry as an “ignition coil,” element 100 actuallyis a transformer. The aforementioned model 8261 ignition coil has a lowinductance primary and provides a 42-43 kV output from its secondarycoil when the primary coil is energized. The secondary coil oftransformer 100 is directly connected (through secondary coil 102B ofboost transformer 102) to one or more electrodes of igniter 101, anotherelectrode of which is grounded.

The string 106 of diodes, each paralleled by a high resistance, limitsthe output voltage of the ignition coil 100 to a single polarity andprevents ringing.

After the breakdown pulse, a trigger signal is applied at 105 to cause afollow-on pulse to be generated. The boost transformer 102 feeds thehigh voltage line (HVL) to igniter 101 with a pulse of current inducedby discharging capacitor 103. Capacitor 103 is charged to a relativelylow voltage such as, for example, about 500V and then discharged throughthe primary coil 102A of transformer 102 to ground through the SCR 105A.

The trigger signals can be generated by any suitable circuit that mayprovide either fixed or programmable parameters.

The igniter electrode(s) connected to the high voltage line are alsoconnected, through a string of diodes 107, and an RC network 111, to alow voltage supply, such as the indicated 500V supply. The resistorvalues in network 111 are set to deliver the simmer current, i_(s)

The ignition circuit of FIG. 8 , it will be appreciated, represents justone way to generate the breakdown voltage and to deliver the initialcurrent and the follow-on pulses of current that are desired. Any othersuitable mechanism may be employed that generates comparable pulsing.For example, a resonant current circuit that could provide oscillatingcurrent pulses, such as sinusoidal current pulses, could be used insteadof the indicated plurality of sub-circuits, each of which generates asingle pulse. Moreover, by proper inversion of polarities of voltage anddiodes, the circuit of FIG. 8 could be used to generate negative pulsesinstead of positive pulses.

Another example of an ignition circuit architecture (in simplified form)is shown in FIG. 9 at 130. Only the basic circuit components are shown,it being understood that a practical implementation may require othercustomary components. Power supply 132 supplies a voltage (termed the“high” voltage for purposes of distinguishing it, only). The voltage ishigh enough so that it can generate, when stepped up by transformer 134,a breakdown voltage sufficient to create a plasma at the igniter (notshown). Power supply is connected to a first end of primary winding 134Athrough a diode 136, to charge a capacitor 138, connected between theother end of the primary winding and ground. A pulse generator 142supplies a train or sequence of pulses. On a first pulse, an outputsignal from pulse generator 142 closes electronically controlled switch144. This action grounds the anode of diode 136, effectivelydisconnecting supply 132 so that it is not short-circuited, and allowscapacitor 138 to discharge through the primary winding, Transformer 134is a saturable-core step-up transformer. The HV supply 132 typically hasan output voltage of a few hundred volts. The closing of switch 144generates a large voltage swing across the transformer primary.Typically, a turns ratio of about 1:35-1:40 may be used in thetransformer, and this will step up the several hundred volt swing on theprimary up to the range of tens of thousands of volts across thesecondary winding, 134B. This latter voltage is sufficient to producebreakdown when applied to an igniter (connected to one end of thesecondary winding, but not shown).

The aforesaid pulse preferably also saturates the core of transformer134.

Due to the core saturation, if a next pulse is supplied by the pulsegenerator 142 before the saturation ebbs totally, such pulse will notgenerate a breakdown-level output voltage on output line 152.

The other end of primary winding 134B, at 154, and one end of acapacitor, 156, are tied to ground via a diode 158. Capacitor 156 ischarged by a “low voltage” (LV) supply through a protective diode 164.When a pulse from pulse generator 142 is received by electronic switch166, node 168 is grounded and capacitor 156 is grounded throughseries-connected diode 172, resistor 174 and switch 168.

Low Voltage supply 162 may typically supply a voltage in the range of0-1000 volts. Capacitor 156 is a large capacitance in a typical ignitionsystem and resistor 174 may be sized to limit the discharge current(pulled through the secondary winding 134 of the transformer) to about50 Amperes (less if a lower current will suffice in the follow-onpulses).

Diodes 182 and 184 merely protect their respective switches from reversepolarity spikes that could be destructive to them.

Supplies 132 and 162 are shown as separate but a single supply may beused in some applications. Also, the terms low voltage and high voltageare not intended to require that the output of supply 132 be at a highervoltage than the output of supply 162, though that is most typical.

Diode 164 is included for the same reason as diode 136, to protect itsassociated power supply from having a short-circuited output when theassociated switch is closed.

Depending on the exact construction of the supplies 132, 162, it alsomay be desirable to place a resistance in series between the one or bothof the supplies and corresponding switch 144 or 166, as applicable, tolimit the output current of the supply and the charging time of thecorresponding capacitor.

Switches 144, 166 may be implemented using various semiconductors, suchas SCRs, IGBTs (especially for switch 144), MCTs and other high voltageswitching elements as now or in the future may exist.

A small capacitor, 159, may bypass diode 158, providing a low impedancepath to ground for rapid voltage changes and protecting diode 158against large reverse spikes.

Other variations are possible. For example, instead of a single pulsegenerator actuating switches 144 and 166, each switch may be actuated bya different pulse generator, or one pulse generator may be employed withdifferent outputs or differently conditioned output signals (possiblyderived from a common signal) driving the switches. Or, one switch maybe used, instead of two switches, as shown in FIG. 10 , referring toswitching element (e.g., MCT) 186. (In FIG. 10 , the resistors R areexpressly shown though they may not be needed, depending on power supplydetails.) If different pulse generators drive each of the switches, theycan be controlled independently and this will permit a variety of modesof operation to be accommodated.

In FIG. 9 , resistor 174 is shown in a dashed-line box, to indicate itis optional. Irrespective of the fact that supply 162 may be set inconjunction with capacitor 156 to control the desired amplitude offollow-on current pulses, all of the energy stored in capacitor 156cannot be transferred to the arc. To sustain a current in the follow-onpulses over the interval of each pulse, the capacitor 156 must bedischarged at a controlled rate. One way to do this is to discharge thecapacitor through a resistor, such as resistor 174. Unfortunately, theuse of resistor 174 results in the dissipation of a lot of the storedenergy as heat. Indeed, more energy may be lost as heat in resistor 156than is expended in the movement of the plasma. Hence this circuitsuffers from inefficient use of energy.

It is possible to improve the efficiency of the circuit and to reducethe heat dissipation by making the switch element 166 a controlledcurrent drainage path. Then, instead of using resistance 174 to limitthe current drain off of capacitor 156, the switch transistor (or likeelement) takes care of that need, providing controlled discharge. Morespecifically, as shown in FIG. 11 , an active switching element (hereindicated as a MOSFET 166′), is connected from node 168 to groundthrough a resistor 192. The voltage across that resistor is sensed as aproxy for measuring the actual current through transistor 166′. Gatedrive logic 194 interposed between the pulse generator and the gate oftransistor 166′, responsive to the voltage on resistor 192, operates thetransistor as a switching regulator, with variable duty cycle and aresulting lower power dissipation than that arising from the use ofresistor 174. Drive logic 194 may be implemented in various ways and mayinclude fixed logic or it may include programmable logic, possiblyincluding a microcontroller to operate the logic. An advantage of usinga microcontroller is that the logic can then be configured to operatethe circuit to perform in the various modes discussed herein—e.g., withor without simmer current.

Note that although the generation of pulses of positive polarity willresult from the illustrated examples of ignition circuits, those skilledin the art of electronics will readily be able to derive therefromignition circuits that will produce negative polarity pulses and evenpulses of varied polarities, should it be desired to have same. It mayalso be desirable that some or all trigger pulses be o polaritydiffering from the output pulses.

The detailed design of the drive logic and the parameters for thebreakdown voltage, follow-on pulses, igniter, etc. will all depend onthe particular engine specifications which the ignition system isrequired to meet. Those requirements, and considerations such as cost,component availability, and so forth will influence component selection,as well. Determination of some of these parameters may require a degreeof experimentation on a model of the engine(s) for which the ignitionsystem or circuit is intended.

Although the problems and their solution have been discussed using justone form of TSI, both apply equally to other TSI designs, using bothparallel and coaxial electrodes.

While certain methods and apparatus have been discussed herein for usewith internal combustion engines operating at high and very highpressures, it will be understood that this technology also can be usedwith traveling spark igniters in internal combustion engines operatingat lower, conventional pressures, or even with conventional spark plugs.The advantages, however, probably will be greatest with traveling sparkigniters.

Also, it should be understood that although a theory of operation hasbeen presented, there are number of simplifying assumptions which mayvery much limit application of this theory. Nevertheless, the invention,as claimed, does produce a working ignition system in a simulated highpressure engine environment, and any simplifications or errors inanalysis will be understood not to detract from the value of theinvention.

Having thus described several aspects of at least one embodiment of thisinvention, it is to be appreciated various alterations, modifications,and improvements will readily occur to those skilled in the art. Suchalterations, modifications, and improvements are intended to be part ofthis disclosure, and are intended to be within the spirit and scope ofthe invention. Accordingly, the foregoing description and drawings areby way of example only.

What is claimed is: 1-6. (canceled)
 7. A method, comprising: applying,to at least a pair of electrodes of an igniter, a voltage of amplitudesufficient to cause breakdown to occur between the pair of electrodes,resulting in electrical discharge current in an initiation regionsufficient to form a plasma kernel adjacent the initiation region; andpassing at least most of the electrical discharge current through aswitching element capable of being switched off with at least 3 Amperes(A) of current therethrough.
 8. The method of claim 7, wherein theswitching element is capable of being switched off with at least 3 to450 A of current therethrough.
 9. The method of claim 8, wherein theswitching element is capable of being switched off with at least 5 A ofcurrent therethrough.
 10. The method of claim 9, wherein the switchingelement is capable of being switched off with at least 5-200 A ofcurrent therethrough.
 11. The method of claim 10, wherein the switchingelement is capable of being switched off with at least 20 A of currenttherethrough.
 12. The method of claim 11, wherein the switching elementis capable of being switched off with at least 40 A of currenttherethrough.
 13. The method of claim 7, further comprising drawing theat least most of the electrical discharge current to and/or from acapacitor.
 14. The method of claim 13, further comprising switching theswitching element to draw the at least most of the electrical dischargecurrent to and/or from the capacitor, passing the at least most of theelectrical discharge current through the switching element.
 15. Themethod of claim 7, wherein the electrical discharge current flowsbetween the pair of electrodes and through the switching element. 16.The method of claim 15, further comprising stepping up a voltage storedin the capacitor to obtain the voltage of amplitude sufficient to causebreakdown to occur between the pair of electrodes.
 17. The method ofclaim 7, further comprising switching the switching element off whilecurrent therethrough is not zero.
 18. A method, comprising: applying, toat least two electrodes of an igniter, a voltage of amplitude sufficientto cause breakdown to occur between the at least two electrodes,resulting in electrical discharge current in an initiation regionsufficient to form a plasma kernel adjacent the initiation region; andpassing at least most of the electrical discharge current through athyristor or an IGBT.
 19. The method of claim 18, comprising passing atleast most of the electrical discharge current through the thyristor,wherein the thyristor is a metal-oxide-semiconductor (MOS) controlledthyristor (MCT).
 20. The method of claim 18, comprising passing at leastmost of the electrical discharge current through the thyristor, whereinthe thyristor is an MCT.
 21. The method of claim 18, further comprisingdrawing the at least most of the electrical discharge current to and/orfrom a capacitor.
 22. The method of claim 21, further comprisingswitching the switching element to draw the at least most of theelectrical discharge current to and/or from the capacitor, passing theat least most of the electrical discharge current through the switchingelement.
 23. The method of claim 18, wherein the electrical dischargecurrent flows between the pair of electrodes and through the switchingelement.
 24. The method of claim 23, further comprising stepping up avoltage stored in the capacitor to obtain the voltage of amplitudesufficient to cause breakdown to occur between the pair of electrodes.25. The method of claim 18, further comprising switching the switchingelement off while current therethrough is not zero.
 26. A circuit,comprising: a capacitor configured to store a first voltage; voltagestep-up circuitry configured to step up the first voltage to a secondvoltage of amplitude sufficient to cause breakdown to occur between atleast two electrodes of an igniter, resulting in electrical dischargecurrent in an initiation region sufficient to form a plasma kerneladjacent the initiation region; and a thyristor or an IGBT configuredfor coupling to the igniter to pass at least most of the electricaldischarge current therethrough.